In a mobile telephone system, uplink transmit power control is performed that controls the power of transmission from a mobile telephone as a mobile terminal unit (hereinafter sometimes referred to simply as the “terminal”) to a base station unit (hereinafter referred to simply as the “base station”) in order to reduce interference and to provide consistent communication quality. More specifically, the base station measures the signal-to-interference power ratio from the signal received from the terminal, and if the measured value is greater than a target value, the base station transmits a command instructing the terminal to reduce the transmit power; conversely, if it is lower, the base station transmits a command instructing the terminal to raise the transmit power. The terminal controls its transmit power by receiving such a command (refer, for example, to patent documents 1 to 4 cited below).
On the other hand, recent years have seen the implementation of a high-speed packet communication standard called HSDPA (High Speed Downlink Packet Access) as a new technology for mobile telephone data communications. With this technology, the modulation and coding scheme (MCS) is dynamically changed in accordance with the radiowave environment of the transmission channel. More specifically, the terminal measures the reception quality of the pilot channel and reports CQI (Channel Quality Indicator), an index corresponding to the reception quality, to the base station at periodic intervals of time. The appropriate modulation scheme and coding rate are selected based on the CQI value.
In HSDPA, two schemes, 16-QAM (Quadrature Amplitude Modulation) and QPSK (Quadriphase Phase Shift Keying), are provided as modulation schemes. While 16-QAM can transmit data at twice the speed of QPSK, it is more susceptible to errors caused by external disturbances. The coding rate is the ratio of the number of information bits to the number of bits after error correction coding, and a coding rate of 8/9, for example, means that eight bits of information become nine bits after error correction coding. Accordingly, as the coding rate becomes larger, the transmission speed becomes higher, but the error correction capability degrades.
Further, an uplink transmit power control method that considers intercell interference is currently under study; in this method, each base station measures received signal power, exchanges interference information with other base stations, and transmits by means of a downlink transmit signal a control signal instructing a terminal located near the boundary of the serving cell to reduce its uplink transmit power.
However, in the HSDPA described above, the transmission speed is determined based on the CQI value. If the uplink transmit power is reduced by the uplink transmit power control based on intercell interference while the transmission speed is maintained constant, the noise level increases, and the error rate correspondingly increases. If the error rate becomes too high, not only the data channel but also the control channel generates an error; when this happens, the base station is unable to correctly demodulate the signal received from the terminal and, in some cases, even becomes unable to recognize the terminal. This increases the possibility that the terminal located near the boundary of the serving cell will in effect be treated as being “out of range.”    Patent document 1: Japanese Unexamined Patent Publication No. 2006-060848    Patent document 2: Japanese Unexamined Patent Publication No. 2003-244069    Patent document 3: Japanese Unexamined Patent Publication No. 2006-222849    Patent document 4: Japanese Patent No. 2830911